1. Field of the Invention
The present invention relates to heat treatment of a low melting point-metal plated steel band, and more especially to a method of and apparatus for continuous overaging treatment of the metal plated steel band to prevent the low melting point metal from being picked up on the surface of a hearth roll in the overaging treatment furnace.
At the overaging treatment temperature of about 250.degree.-500.degree. C., metal is picked up on the hearth roll surface especially when the temperature is more than 70%, of the melting point of the metal, which is indicated by absolute temperature. Thus, metals or alloys having a melting point of less than about 830.degree. C. must be considered. Such low melting point metals which may be considered for metal plated steel band include zinc, aluminum, tin, lead, alloys containing two or more of these metals, alloys of one or more of these metals and iron, and alloys consisting mainly of one or more of these metals and small quantity of other metals, for example, alloys consisting mainly of zinc and small quantity of one or more of other metals, such as cadmium, lead, magnesium and iron, etc.
In the specification, zinc plated steel band is described as a suitable application, however, as to other metals or alloys the same consideration is applied. Further, the invention relates to steel band surface metal pick up phenomena, so that methods of plating or depositing the metal on the steel surface do not change. For illustrating the metal pick up phenomena, hot dip plating is described in the specification. Other methods, e.g. electroplating, chemical plating, vapor depositing, and ion plating can be applied.
2. Description of the Prior Art
Zinc plated steel plate which is manufactured by in-line anneal type hot dip zinc plating apparatus is quenched after in-line annealing so that the steel is hardened somewhat compared with cold rolled steel plate. As a remedy, by overaging treatment process the steel plate is annealed to precipitate solid solution carbon over saturated in the matrix to grain boundaries. Such overaging treatment is applied to hot rolled or cold rolled steel plate of carbon content 0.01-0.2% by weight and zinc plated at one or both surfaces. However, conventional batch type post annealing is not suitable for mass production. Thus, it is desirable to treat the steel plate by in-line overaging treatment process, i.e. after the steel band is plated in a zinc melt bath, the belt is progressively passed in an overaging furnace.
To successfully perform the in-line overaging treatment process, the steel belt temperature range may be 300.degree.-410.degree. C., atmosphere gas temperature may be 250.degree.-430.degree. C., heat treatment period may be 90-360 sec., and line speed may be 20-180 m/min. Thus, a great many hearth rolls are necessary to perform such long heat treatment period as such high speed.
As the melting point of zinc is 419.degree. C., steel or iron roll, alloy roll and ceramic roll, which are generally used in heating furnace, reducing furnace and cooling furnace of an in-line anneal type hot dip zinc plating plant, can not be used as hearth rolls in an overaging treatment furnace.
The reason is as follows: On the surface of the hearth roll in the overaging furnace, zinc is supplied from the zinc plated steel band surface and is deposited as zinc blocks. Thus, the surface of the zinc plated steel band which passes around the hearth roll for about a half circle under tension forms many small recesses.
The principle that zinc is deposited and progresses on the surface of the hearth roll in the overaging furnace is not clearly determined. According to research of the inventors of the present invention, on a Cr-Ni or Co base alloy roll surface, zinc physically attached on the surface is chemically combined with the roll surface within a short period and can not be removed by physical methods, in a temperature range of the overaging furnace. Zinc chemically combined with the roll surface has more tendency to attach other zinc, than a roll surface which is not attached with zinc, so that zinc progresses selectively to form a projected zinc block.
As to a ceramic roll, zinc is attached in small recesses of the ceramic roll surface and the attached zinc progresses as before. Also, ceramic coating forms a network of cracks, and a small part of the metal roll surface is exposed in the overaging atmosphere. In the very small area, zinc is squeezed into the area by relative movement between the roll and the steel band surface, and forms chemically combined strong zinc deposit.